Pipettes and pipette tips come in many configurations. Seals between the pipette and the tip can be formed in several different ways: two conical surfaces coming together, raised rings forming annular pressure points, and a blunt corner or radiused corner forced into a conical tip. To insure a good seal, a technical must insert the pipette into the pipette tip with enough force to affect a seal by displacing plastic in the tip. In some instances this can be considerable.
One solution is proposed in U.S. Pat. No. 4,748,859 to Magnussen, Jr., et al. A disposable pipette tip member is disclosed having three inner coaxial annular sealing bands spaced axially from an open proximal end for receiving a conical pipette tip mounting shaft. The first two sealing bands are relatively resilient and simultaneously engage, guide and laterally support the pipette shaft as it enters the tip member to form annular fluid-tight seals with the shaft. The third band is relatively rigid and upon engagement with the shaft, forms a third annular fluid-tight seal and a controllable stop for the shaft such that the tip member is seated on the shaft adjacent the pipette tip ejector mechanism.
Removal of the tip can be difficult as well, and over many uses during a day, can cause fatigue and even injury over time. A need exists for a method for producing a pipette tip that can be inserted and ejected with a minimal force, maintain a good seal, and provide for a good fit on a variety of pipettes.
Also, a need exists for a method of forming relatively large and resilient rings on the internal sealing surface of the pipette tip. Typically, the size of an internal ring is limited since it is xe2x80x9cundercutxe2x80x9d in the mold that forms the tip. In order to remove the part from the mold, it must expand for the ring to be released from the groove on the mold core that forms it. This limits its size, and can xe2x80x9csmearxe2x80x9d the ring as it is removed.
The present invention is directed to a pipette tip member for releasably mating with a pipette shaft. The tip member comprises an elongated tubular receptacle having a central axis, a protrusion on an inner surface of the receptacle, and a recess on the inner surface adjacent the protrusion. The tip member can have a generally conical shape and the inner surface is tapered from a rear opening at a proximal end to a tip opening at a distal end.
In one embodiment the protrusion extends around the inner surface to form an annular sealing ring and the recess extends around the inner surface to form an annular groove.
The protrusion preferably has a V-shaped axial cross section. In one embodiment the protrusion has first and second generally straight edges that intersect at an angle between about 30xc2x0 and about 120xc2x0, preferably between about 45xc2x0 and about 90xc2x0. In another embodiment, the first and second edges intersect at an angle of about 60xc2x0.
The recess preferably has a generally circular axial cross section and at least a portion of the recess is collinear with at least a portion of the protrusion. In one embodiment the tip member extends from a rear opening at a proximal end to a tip opening at a distal end and the recess is disposed proximal to the protrusion in the axial direction.
In one embodiment the recess extends radially outward from the inner surface and has a depth and the protrusion extends radially inward from the inner surface and has a height and the recess depth is about as great as the protrusion height. In another embodiment the recess depth is greater than the protrusion height.
The present invention is also directed to a method of forming a tubular pipette tip member, comprising the steps of: providing a mold including a mold core with a conical shape and including a mold protrusion disposed adjacent and proximal to a mold recess, introducing mold material into the mold; and removing the mold material by forcing the mold material off of the mold core. In one embodiment, during the step of removing the mold material, the mold material is forced radially outward by the mold protrusion and the tip protrusion is lifted radially outward from the mold recess. In yet another embodiment, during the step of removing the mold material, the tip protrusion is spaced from the mold core after tip protrusion is lifted out of the mold recess.